CAD systems are widely used in the design of mechanical or electronic parts or other objects and there are several commercially available systems of that type. A common characteristic of these systems is that the objects are designed by the user in an interactive mode, that means that the body is displayed on a screen as far as it has been designed by the user and the user can enter commands by means of which additions and modifications of the already existing structure can be made. For displaying the geometric object under design, typically a cathode ray tube (CRT) or a liquid crystal display (LCD) is used. The object is displayed in an isometric view, whereby the displayed object may additionally be illuminated by imaginary (i.e., computer generated) light sources so that the three-dimensional impression is further improved. As an alternative to the perspective representation on the screen, a two-dimensional or cross-sectional view of the object under design can be displayed. Some known CAD systems also allow to rotate the displayed object around different axes so that the user can look at the object from several directions. The commands for supplementing or modifying are entered by suitable input means, preferably a computer mouse, a graphics tablet or a light pen. When the editing process is finished and the object to be designed has assumed its final shape, a corresponding hardcopy can be produced by a printer or plotter. Furthermore, a punched tape or a magnetic tape can be produced from the data representing the object, and this can then can be used directly by a numerically controlled machine tool for manufacturing the desired physical object.
An example for a commercially available 3D CAD system is the Hewlett-Packard PrecisionEngineering SolidDesigner Version 1.0. A description of a 3D CAD system is also known from I. C. Braid: "Notes on a geometric modeller" C A.D. Group Document No. 101, University of Cambridge, June 1979. This prior art document will subsequently be referred to as the Braid reference. A further description of a 3D CAD system is known from the article by I.C. Braid et al.: "Stepwise Construction of Polyhedra in Geometric Modelling", in K. W. Brodlie (Ed.): "Mathematical Methods in Computer Graphics and Design" Academic Press, 1980
In section 4.9 of the Braid reference, a basic discussion of chamfering, the replacement of a straight edge by a small flat face, is given. At the end of the Braid reference, it is stated that the pressing problem of fillets and blends has unfortunately remained unsolved.
In the meantime, several CAD systems have been developed, for example the above mentioned system by Hewlett-Packard, which have the capability to perform blending of edges and, as a special case of blending, chamfering of edges. In the known CAD systems, the user can select an edge which is to be blended by positioning a cursor at this edge using, for example, a computer mouse. When the edge has been selected, the user can enter a radius for the blend curve or, in the case of chamfering, the distance of the chamfer from the original edge. The computer then calculates the shape of the blended or chamfered object and displays it on the screen.
Existing CAD systems, however, are not satisfactory in all respects since they are not able to perform blending for all desirable magnitudes of the blending radius and for all shapes of the represented object. In the practical work with a CAD system, it frequently occurs that the blending of a specific edge would require that other edges or even entire faces of the geometric object are removed. In particular, if the object has many short edges and the blending radius or the chamfer is large, the blending may reach over several edges and faces of the original object and require the removal of such edges and faces from the geometric object. As a simple example, a cube is considered, a vertical edge of which is to be blended. If the blending radius is sufficiently large, a major portion of the cube would be cut away, leaving only a corner piece of the original cube with a cross section corresponding to a sector. In order to generate this corner piece, all edges of the cube which intersect the original edge to be blended, i.e., the four horizontal edges meeting the original edge, have to be removed. Furthermore, the two horizontal faces adjacent the original edge have to be completely removed. In practical design tasks, the objects of interest are more complex than just a cube and the problem of removing edges and faces becomes more complex.
The problem with the present CAD systems is that they are not able to perform the task of removing edges and faces for all possible varieties of the geometry of the object under design and of the blending parameters (blending radius, chamfer). Some CAD systems do have a limited capability of removing individual edges; such edge removal, however, only works for simple geometric objects. With more complex geometric conditions, the CAD system produces an error message on the screen that the blending operation cannot be performed. In any case, applicant is not aware of a CAD system which is able to remove entire faces of the geometric object. Due to these limitations, the selection of the blending radius is severely limited in prior art CAD systems. As a consequence thereof, parts of the geometric model or even the whole model have to be constructed anew, whereby short edges which could lead to the mentioned problems with blending, have to be avoided. This leads to a lengthy design process and is thus costly.